I don't know Assassination Classroom very well. I do know I really like Nagisa's design. Its a nice example of a feminine looking guy without going overboard and the cuteness works well in contrast with Nagisa's job. Its always somewhat annoying if the main character is ideal for getting kidnapped but never gets tied up and gagged. There's even a part where people out to kidnap some girls just knock him out with the other boys, despite him being mistaken for a girl being part of Nagisa's character.
But apparently Nagisa did get tied up and gagged in some silly spin off where the characters are part of some sort of jrpg world. Nagisa gets mistaken for a girl, kidnapped and gets to play damsel in the tower for a bit. The ropes are pretty quickly replaced with a crossdressing joke which is unfortunate but eh, lets just count our blessings.
Koro sensei quest: episode 9
Assassination Classroom: Nagisa's kidnapping
Moderator: Suicide